This was posted by Rod Henrickson on a gunsmithing forum. He was talking about using a grease gun to push stuck bullets out of a barrel.

I have used a common Alemite high pressure grease gun to pump out probably between 50 and 100 stuck bullets over the last 30 years. While the bullet will seal on stiff grease, it won't seal on air. The air simply leaks by the bullet as fast as you can work the handle on the grease gun. I have never yet seen a bullet seal the bore air tight.

OK, so slowly pushing a bullet out is a lot different than rapidly pushing a bullet out. We know that black powder usually has enough "oomph" to obturate a soft bullet to form a gas seal. The trick is to build up pressure rapidly, before the gas can leak past the as-yet un-obturated bullet. That may or may not happen with smokeless powder and hard bullets. In theory the pressure is plenty high enough to obturate the bullet, but it takes a while for the pressure to peak and in the meantime gas cutting may already have taken place.

Enter bullet lube. It helps form a gas seal, just like the grease Rod uses to push out stuck bullets. I've long theorized that the main function of bullet lube is to help form a gas seal. It probably does lubricate, as well, but that's a secondary function.